The use of heat shrinkable thermoplastic film as flexible packaging material for vacuum packaging perishable food mass is well-known. This type of film is relatively thin, e.g. less than 4 mils, so itself is not suitable for packaging bone-in food mass such as meat. For example, attempts to use such thin film in bag form to package bone-in sub-primal rib beef cuts are generally unsuccessful because the bone punctures the film. The puncture problem is compounded by external abrasion between adjacent packages when they are transported in containers subject to intransit vibration and movement during loading and unloading.
To alleviate this problem the most common practice was to use cushioning materials such as paper, paper laminates, wax impregnated cloth, foam and various types of plastic inserts inside the bag over the bone-in section, as for example described in Selby et al. U.S. Pat. No. 2,891,870. This approach was only a partial solution because the inserts tend to slide during usage and are labor-intensive.
Another approach was to adhere a puncture guard in the form of a patch on the outer surface of the heat shrinkable bag. One form of patch was a plurality of oriented sheets which are laminated in cross-oriented relationship, as for example described in Conant U.S. Pat. No. 4,239,111. However, in actual use the manufacturer reported that the non-heat shrinkable patch, which was adhesively bonded to the bag outer surface, tended to delaminate when the evacuated bag was heat shrunk around and onto the bone-in food mass outer surface. Another complication with cross oriented patches, such as those formed of high density polyethylene manufactured from material obtained from Van Leer Plastics B.V., under the trademark VALERON.RTM., is that the material is relatively stiff and does not readily conform to the contours of a bone-in food mass containing bag. According to Kuehne U.S. Pat. No. 4,534,984 this problem may be overcome by the additional process steps of forming longitudinal lines of weakness as for example by slitting or serrating, then folding the patch along these lines.
To overcome these problems, Ferguson U.S. Pat. No. 4,755,403 describes a patch bag combination wherein a particular type of heat shrinkable patch is bonded by adhesive to the outer surface of the heat shrinkable bag. The shrink properties of the bag and patch are matched so that on heating, the patch shrinks with the bag and thereby reduces the tendency of the patch to delaminate from the bag. Because the patch is relatively thick, for example 5 mils, it is most conveniently manufactured as a multilayered tube with self adhering inner surfaces. Accordingly, when the tube is collapsed on itself the inside surfaces of the inner layers "block" or adhere to each other and a relatively thick heat shrinkable patch is formed.
More specifically, the aforementioned U.S. Pat. No. 4,755,403 describes a patch formed from a tube comprising an outer layer of 87% linear low density polyethylene (LLDPE), 10% ethylene vinyl acetate (EVA) having 9% vinyl acetate (VA) content, and an inner layer comprising EVA with 28% VA content. Because the inner layer must be self adhering, the tube must be extruded with powder such as starch particles on the inner layer inside surface to prevent adhesion during extrusion. This is necessary because the primary tube must be reinflated to form the trapped or secondary bubble if the tube is to be biaxially oriented by this method. When the resulting oriented tube is collapsed, the starch particles are sufficiently spread apart by the two way stretching and thinning of the film, that the collapsed tube becomes self adhering.
Patent '403 also teaches that irradiative cross linking of the patch is necessary to strengthen the tube sufficiently to permit inflation as a bubble for biaxial orientation. Accordingly, the irradiation step must be performed on the relatively thick primary tube, and relatively high power is needed for this because of the thick-walled tube.
It will be apparent from the foregoing that the patch bag of Patent '403 is relatively expensive to manufacture because of the need to use high VA content EVA (for self adhesion), the need for multiple layers, the need for powdered starch as an antiblock, the high power consumption resulting from irradiation of a relatively thick patch, and the need for biaxial orientation. Moreover, the manufacturing process requires adhesive application to either or both the patch inner surface and the bag outer surface, careful placement of the patch on the bag or rollstock surface for proper mating of adhesive-coated surfaces, pressure contact and elevated temperature curing of the adhesive bond.
There are also inherent functional limitations on the heat shrinkable patch-bag combination. Since the patch biaxially shrinks to about the same extent as the substrate bag, a substantial proportion of the as-applied patch surface area does not perform the guard function when heat shrunk. This means that whereas a protruding bone area of food mass may have been covered by an overlying patch when placed in the bag, when the patch-bag combination is heat shrunk around the food mass a significant portion of the bone area on the perimeter of the non-shrunk patch may be no longer covered by the non-shrunk patch. For example, if the original patch is square and 10 inches on each side and the shrink is 25% in both directions, the cross-sectional area of the heat shrunk patch is only about 56% of the original surface.
The prior art has taught that for some applications, thermoplastic surfaces may be made self adhering by exposing the surfaces to corona treatment and then pressure contacting the surfaces. For example, Shirmer U.S. Pat. No. 4,605,460 discloses a high barrier shrink film wherein the EVA surfaces of a hot blown melt oriented high oxygen barrier film and a stretch oriented base film are each corona treated and then contacted between nip rolls for lamination. However, to the best of applicants' knowledge corona treatment has not been used in patch bag construction to bond the patch and the bag, probably because of the high abrasion/delamination forces experienced by the patch in commercial use.
One object of this invention is to provide an improved patch bag article for enclosing bone-in food products.
A specific object is to provide an improved patch bag article wherein the patch need not be irradiated to perform its intended function.
Another object is to provide an improved patch bag article comprising a non-heat shrinkable patch which does not delaminate from the evacuated bag when the latter is heat shrunk around bone-in food mass.
A further object is to provide an improved patch bag article comprising a non-heat shrinkable patch, heat shrinkable bag article which does not require an adhesive therebetween, yet with a patch-bag bond so strong that substantially no delamination of the patch occurs when the evacuated bag is heat shrunk.
Still another object is to provide an improved food package comprising a heat shrunk, evacuated and sealed bag containing bone-in food mass and a non-delaminated non-heat shrinkable patch bonded to the bag outer surface without a separate adhesive.
A still further object is to provide an improved method for packaging bone-in food mass in an adhesive-free heat shrinkable bag - non-heat shrinkable patch article by evacuating and sealing the food mass - containing article, and heat shrinking the package without delamination of the patch.